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Ozone hole and the upper atmosphere

Polar mesospheric clouds (also known as noctilucent clouds) are most often observed in the summer months at high latitudes (greater than 50 degrees) above both the northern and southern hemispheres. Illuminated by sunlight when the lower layers of the atmosphere are in the darkness of Earth’s shadow, the clouds appear bright and cloudlike at twilight. (Image courtesy NASA Earth Observatory.)

December 23, 2010 | The impacts of the Antarctic ozone hole extend upward as well as downward, according to a new modeling study from a team of NCAR scientists. They found that the ozone hole appears to be affecting temperature and circulation patterns in the mesosphere, leading to differences in the qualities of polar mesospheric clouds.

The mesosphere is the layer of the atmosphere that begins around 50 kilometers (31 miles) above Earth's surface, just beyond the stratosphere. Observations have detected differences in altitude and brightness between polar mesospheric clouds (clouds made of ice crystals in the upper mesosphere) in the Northern Hemisphere and those in the Southern Hemisphere. Scientists have suggested various mechanisms for explaining this difference.

The new study, led by atmospheric chemist Anne Smith, points to the ozone hole in the stratosphere above Antarctica. By running simulations with the Whole Atmosphere Community Climate Model (WACCM), the team found that the ozone hole causes a decrease in temperature in the lower stratosphere that persists into the summer. This is accompanied by wind changes that modify the upward propagation of small-scale waves, which in turn alter the atmospheric circulation in the mesosphere above the Southern Hemisphere.

The team found that this hemispheric asymmetry was small before 1980 but increased at about the same time as the onset of the Antarctic ozone hole. A model run with no ozone loss showed no increases in the hemispheric asymmetry of mesospheric circulation and temperature, confirming that ozone loss is a likely cause of the hemispheric differences.

Smith cautions, however, that the decrease in stratospheric temperature due to the ozone hole shown by the model is larger than observed, so it is likely that the simulated response in the mesosphere is also too large.

“As the ozone hole recovers in upcoming decades, these trends in mesospheric temperature and circulation may reverse,” she adds.